Method for conducting an electrochemical oxidation



April 28, 1964 E. B. LANCASTER ETAL 3,131,137

METHOD FOR CONDUCTING AN ELECTRO-CHEMICAL OXIDATION Original Filed Dec. 15, 1959 2 Sheets-Sheet '1 EARL B- LANCRSTER HOWARD F- CONWAY FRANK C. WOHLKRBE.

I ATTORNEY April 28, 1964 E. B. LANCASTER ETAL 3,131,137

- METHOD FOR CONDUCTING AN ELECTRO-CHEMICAL OXIDATION 2 SheetS -Sheet 2 Original Filed Dec. 15, 1959 n u F .0

omN n xO uhdc m0 PZMOK 2 TIME, HOURS EARL a. LANCASTER H QWAR F. couwm FRAms c WOHLRABF- ej j 03 7 I [51" WW ATTORNEY United States Patent 1959, Ser. No. 859,824. 1962, Ser. No.

2 Claims. (Cl. 2%4-82) (Granted under Title 35, US. (jade (1952), see. 26%) A nonexclusive, irrevocable, royalty-free license in the invention herein described, throughout the world for all purposes of the United States Government, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States of Arnerica.

This application is a division of application bearing Serial No. 859,824, filed December 15, 1959, now abandoned.

This invention pertains to an improved method having surprisingly superior performance characteristics which provide great economics in the oxidation of sodium iodate to sodium periodate for the out-cell oxidation of polysaccharides such as starch, cellulose, and the like to the substantially fully oxidized form, e.g., dialdehyde starch. With but minor adaptation to provide a required active circulation, electroyltic cell in which the present process is carried out is equally superior for the in-cell oxidation of polymer polysaccharides by the continuous electrolytic reoxidation of iodate to periodate.

Our invention is an improvement over the prior art shown in US. Patent No. 2,648,629 to Dvonch et al., US. Patent No. 2,713,553 to Mehltretter et al., and in Mehltretter et al., Ind. Eng. Chem., 49, 350 (1947).

In the prior art it has been assumed that the conductivity of the electrolytes in an electrolytic cell containing a diaphragm is much greater than that of the diaphragm and that the relationship of the diaphragm to the electrodes is not critical. Thus, the prior art shows a conventional periodate-starch oxidation cell as comprising ceramic thimbles containing a sodium hydroxide catholyte in each of which is suspended a steel rod cathode. The ceramic thimbles are symmetrically disposed relative to a midline anode which comprises a flat plate partially of lead dioxide on lead, the anode being suspended in an anolyte comprising a starch slurry and iodic acid formed by reaction of sodium iodate and sulfuric acid. In an alternative prior art arrangement, a central cathode symmetrically situated between two flat plate anodes has been employed. The rate at which such cells produce active oxygen has been thought to be related to the current passing through the cell, the area of the lead surface, and the activity of the lead surface up to a design limit current flow value at which point the oxygen produced would not all be active, and part of it would pass out of the cell to the atmosphere. In other words, it has been thought in the prior art that the current density must be held below a certain low value for the oxidation to be efficient. The Mehltretter article, cited above, clearly teaches that the current efiiciency is inversely related to the current density and that it falls sharply even with small currents.

We have now discovered that sodium iodate can be much more efliciently oxidized to the periodate for the out-cell or in-cell oxidation of polysaccharides such as starch to the substantially fully oxidized form such as dialdehyde starch, with great savings in oxidation time, electricity, anode costs, and economy of required fioor space. More specifically, we have discovered that the above and other advantages, which together very materially lower the present hi cost of periodate-oxidized dialdehyde idhlli? Patented Apr. 28, 1964 ice starch, are obtained if the oxidation is carried out in a concentric or annular type of electrolytic cell having a certain critical relationship of a cylindrical porous ceramic diaphragm and circumferential anode.

We have found that iodic acid can be oxidized to periodic acid and, concurrently, starch to dialdehyde starch about twice as eiiiciently and in one-third the time in a concentric cell in which the distance of a suspended, thimble-like porous ceramic diaphragm containing the catholyte and having suspended therein a steel rod cathode, is uniformly no more than one inch and is preferably between 0.7-0.5 inch distant from the inner wall of a concentrically disposed annular anode, which can be made of ordinary lead pipe rather than of one percent silver-lead alloy or sulfuric acid-activated lead dioxide on lead. For best results it is preferred that the surface area of the cathode be on the order of one-tenth that of the internal surface of the anode. As a practical matter the size of the anode is only limited by the available sizes of porous ceramic diaphragms. While the height (length) of the components is not critical and is limited only by that of the available diaphragms and by the necessity for efiicient, rapid circulation and mixing of the starch to keep the latter from more than momentary contact with the diaphragm, a battery of cells can be easily arranged to occupy a minimum of floor space. The cell in which the process of the present invention is carried out is illustrated in the accompanying drawings in which FIGURE 1 is an elevation, partly in section, the complete assembly;

FIGURE 2 is a graphic comparison between the amount of iodate oxidized plotted against time when using the cell of the present invention (curve A) and when using prior art cells (curve B); and

FIGURE 3 is an enlarged section of the upper portion of the cell showing, in detail, the means of assembling the several elements.

FIGURE 1 represents an elevation, partially in section, of the cell of our invention, with means provided for connecting a high speed circulating pump and cooling sump required for the direct in-cell periodate oxidation of starch. The anode 1 is a commercial lead tube 2.5 inches in outside diameter, having a wall thickness of /8 inch, and bearing near the top a side-arm tube of convenient optional diameter corresponding with that of an aperture in the bottom of a tapered lead extension 2 joined to the bottom of the anode in any suitable manner for a circulation connection to a pump (not shown). As shown in detail in FIGURE 3, a porous ceramic diaphragm 3 is suspended uniformly central to the anode within the critical distance from the inner surface thereof by means of rubber ring 5. A inch iron rod cathode 4 is in turn suspended at a uniform distance from the inner surface of the diaphragm by means of rubber ring 6. The diaphragm and cathode are immobilized at their respective positions by means of the said rubber rings, the inner ring also having a small eccentric bore and tube 7 inserted therein for venting hydrogen. The cell thus comprises an annular active electrolyte compartment of critical thickness, uniformly disposed in relation to an inactive catholyte compartment, initially containing as the standard catholyte a 5 percent solution of sodium hydroxide, the concentration of which increases during the course of the electrolysis to between 1640 percent.

As shown by the results contained in Table I, FIGURE 2 and the examples, our cell provides significantly higher current efficiencies with large currents in sharp contrast with analogous cells of the prior art which are limited to the use of very low amperages. In a cell of our design, currents of 10-20 or more amperes are only moderately less efficient than are very small currents and current densities, and this permits a much more efficient and rapid showing reoxidation to periodate and oxidation thereby of the polysaccharide. Also, the shortened residence time lessens the degradation of small amounts of starch by unavoidable exposure to leakages of alkali and iodide through the diaphragm.

Without wishing to be held accountable for the following theory, we believe that the unusual spacing of our diaphragm in some way promotes the formation and preservation of a particular lead oxide surface on the anode, which formed surface is unusually active in producing active oxygen and that the very large current density at the cathode inhibits the formation of active hydrogen which could diffuse into the anolyte solution and reduce some of the periodate formed there, thus lowering the apparent emciency.

Table I shows that annular cells having the critical anode-diaphragm spacing of our invention handle very large currents and anode current densities with much lower loss of efficiency than the cells of the prior art.

FIGURE 2 (curve A) shows the oxidation of 200 gms. sodium iodate to sodium periodate in a 300 sq. cm. anode area annular cell having an anode-diaphragm distance of 0.5 inch, employing a current of 20 amps. (anode current density of .066 amp/sq. cm.). This is compared with a prior art flat plate anode cell of the same anode area (curve B) at the same current and anode current density.

The following examples are presented to further illustrate the practice of our invention. Although we have shown our invention as related to oxidations, it is clear that by reversing the polarity of the cell it could be employed for electrolytic reductions.

Example I The cell consisted of a porous ceramic thimble as catholyte compartment 10 in. long by 1.5 inches in outside diameter, with a round bottom and walls 4; inch thick. The cathode was an iron rod, inch in diameter, centrally disposed within the catholyte compartment and reaching to within about an inch of the bottom of said compartment. The anode was a Ms inch thick commercial lead tube, 2.5 inches in diameter and 9.5 inches long to which a conical lead bottom extension was soldered. At the center of the conical extension, a 0.5 inch diameter lead tube was afiixed for circulating pump attachment and, at a point 3.5 inches from the top of the anode, a corresponding 0.5 inch diameter lead tube was affixed. The anode was disposed concentrically with the cathode and diaphragm so that an annular space (excluding the bottom) of 2.5 inches outside diameter (in flat plane approximately 0.5 inch wide by 6 inches long) was provided for circulation of the anolyte. The cell was provided with a pump (not shown) for circulating the anolyte at a rate of about 600 cc. per minute, a holding vessel or sump (not shown) for containing the main part of the anolyte charge, and an anolyte cooling means (also not shown} which was, in this case, a glass condenser through which the anolyte flowed after it left the cell and before it entered the sump. The sump was filled with 3 liters of distilled water in which 200 grams of sodium iodate were dissolved. The anolyte was circulated through the cell and 20 amps. were passed through it, thus representing an anode current density of about 0.065 amp. per square centimeter. At the end of 3 hours, 84 percent of the sodium iodate had been oxidized so that only 16 percent of the original charge remained unoxidized, and the pH of the solution had been reduced to below 2.0. This means that the oxidation proceeded at an anode current efficiency of about 76 percent. To one skilled in the art, this is an unusually high efficiency at a current density of 0.065 amp. per square centimeter, it being generally thought that this efliciency can only be obtained using anode current densities of the order of 0.012 ampere per square centimeter. Thus it can be said that the activity of the lead has been increased by as much as 500 percent.

Example 2 The cell consisted of a ceramic thimble as catholytc compartment arranged in conjunction with a lead anode to form an annular anolyte compartment as described in the specifications and Example I. In this case, the thimble had a /2 inch thick wall, was 21 inches in length, and 2.5 inches in outside diameter, with a fiat bottom. The anode forming the outer Wall of the anolyte compartment was made from /8 inch thick lead containing 1 percent silver, and was 3.5'inches in diameter and 18 inches in length, with a flat bottom. The inlet was a /2 inch diameter commercial lead tubing afiixed so that the inlet stream entered at the side of the anolyte compartment near the bottom in a tangential manner. The outlet was a one inch by one inch weir cut in the top of the lead tube anode to which was aihxed a spout for conducting the anolyte about 6 inches away from the cell, whereupon it could freely fall into a sump provided for the main body of anolyte. The cathode was a inch (i.p.s.) iron pipe fitted at the top With an ear for facilitating the negative electrical connection. The positive connection was made to a brass bar soldered to the outside of the anode, and running lengthwise down it. Anolyte was circulated at about 1.5 gallons per minute, and was cooled in the sump by a coil through which water was passed. The sump was filled with 3 gallons of anolyte containing 888 grams of sodium iodate and this was pumped through the anolyte compartment while maintaining 100 amperes through the cell. This is an anode current density of about 0.078 amp. per square centimeter. Ninety percent of the iodate was oxidized to periodate in 3 hours at a current efiiciency of about 71 percent. These results could be achieved by a conventional cell only if it employed three times this anode area and was operated in accordance with the best previous practice. Such a cell would have about 15 times the volume of our invention.

Example 3 The cell as described in Example 1 was operated to oxidize starch by adding 90 grams of starch to 350 cc. of a solution containing 23 grams of sodium iodate, 3.5 ml. of sulfuric acids, and 52 grams of sodium sulfate. The slurry was pumped directly from the bottom of the cell to the top of a glass condenser, through which it fell into the top of the cell in a thin film. The side opening on the cell was closed with a stopper for this operation. To minimize the amount of starch inventory and expedite the oxidation, no sump was used. A current of 5 amperes was passed through the cell at 4.4 volts for 5.0 hours, and a sample of the slurry removed and the starch recovered from it by washing with water. The starch had been oxidized to 86 percent dialdehyde content, indicating a current eliiciency of 73 percent. The usual results with other cells give this oxidation in 48 hours with a current etficiency of 5 0 percent.

Having thusly disclosed our invention, we claim:

1. A method of electrochemically oxidizing sodium iodate to sodium periodate at a current efficiency of about from 60 to 75 percent comprising: (a) introducing a solution of sodium iodate into an annular anolyte chamber of a coaxial cell, said cell comprising an outer elongated cylindrical anode, a coaxially disposed elongated cylindrical porous diaphragm within the anode, and a coaxially disposed elongated cathode Within the diaphragm, said anolyte chamber being defined by the inner wall of the anode and the outer Wall of the diaphragm, said inner Wall and outer wall being uniformly spaced about from 0.5 to 1.0 inch apart; (1]) continuously force-circulating the solution of sodium iodate and reaction products into and out of the anolyte chamber; and (c) passing a current of about from 5 to 75 amperes through the solution, the dimensions of the anode and cathode being such as to provide an anode current density of about from 0.06 to 0.078 10 ampere/ sq. cm.

2. The method of claim 1 wherein the circumference of the cathode is about 0.1 of that of the anode.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Trans. Electrochemical 500., volume 71, pages 505 to 517 (1937), Bradt et a1. 

0.5 TO 1.0 INCH APART; (B) CONTINUOUSLY FORCE-CIRCULATING THE SOLUTION OF SODIUM IODATE AND REACTION PRODUCTS INTO AND OUT OF THE ANOLYTE CHAMBER; AND (C) PASSING A CURRENT OF ABOUT FROM 5 TO 75 AMPERES THROUGH THE SOLUTION, THE DIMENSIONS OF THE ANODE AND CATHODE BEING SUCH AS TO PROVIDE AN ANODE CURRENT DENSITY OF ABOUT FROM 0.06 TO 0.078 AMPERE/SQ.CM.
 1. A METHOD OF ELECTROCHEMICALLY OXIDIZING SODIUM IODATE TO SODIUM PERIODATE AT A CURRENT EFFICIENCY OF ABOUT FROM 60 TO 75 PERCENT COMPRISING (A) INTRODUCING A SOLUTION OF SODIUM IODATE INTO AN ANNULAR ANOLYTE CHAMBER OF A COAXIAL CELL, SAID CELL COMPRISING AN OUTER ELONGATED CYLINDRICAL ANODE, A COAXIALLY DISPOSED ELONGATED CYLINDRICAL POROUS DIAPHRAGM WITHIN THE ANODE,AND A COAXIALLY DISPOSED ELONGATED CATHODE WITHIN THE DIAPHRAGM, SAID ANOLYTE CHAMBER BEING DEFINED BY THE INNER WALL OF THE ANODE AND THE OUTER WALL OF THE DIAPHRAGM, SAID INNER WALL AND OUTER WALL BEING UNIFORMLY SPACED ABOUT FROM 